Morphogenesis of supramolecular structure in biological systems is an important but poorly understood aspect of growth and development. Recent research from our laboratory and others indicates that several examples of macromolecular assembly cannot be explained simply by self-assembly of structural proteins, and that accessory or catalytic proteins also may be involved in many assembly reactions. In pursuing our study of such reactions, we shall continue to exploit the bacteriophages T4 and T7 as instructive model systems for macromolecular assembly. We shall use biochemical genetic, serological, and structural approaches to investigate several problems of assembly, in particular: 1) The roles of accessory proteins in T4 tail-fiber assembly, and 2) The mechanism of DNA packaging into the head of bacteriophage T7. 1) T4 tail fibers, the adsorption organelles of the phage, are composed of only four structural proteins, yet their assembly and attachment to the tail baseplate requires the functions of four additional gene products that appear to play catalytic roles. We shall characterize these functions using the approaches mentioned above, with the goal of elucidating the biochemical mechanisms of all four gene products in the assembly process. 2) Encapsulation of nucleic acids in a protein coat is an essential feature of all viral life cycles that is still understood. For bacteriophage T7 this process has been demonstrated in extracts of infected cells, and therefore can be investigated biochemically as well as genetically and structurally. Characterization of intermediates has led us to a new hypothesis for the packaging mechanism which we shall test by further study of both the in vivo and in vitro reactions. Our goal is to understand the energetics and mechanism of the DNA packaging process by biochemically defining the steps in the in vitro reaction.